Tweedledum and Tweedledee

Terbium and erbium

walked into a bar and ordered

oxygen-potassium cocktails.

They asked for the senior discount.

The bartender said OK, OK,

but not for your friend yttrium;

we know he’s only 39.

The discovery of a new earth (an oxide), such as erbia,
was assumed to be equivalent to the discovery of a new element, erbium,
even though it was impossible, at first, to isolate the element,
and even though the element might be accompanied by traces of other earths,
which had proven to be the case with yttria,
and would prove to be the case with terbia and erbia.